The plan: the analytic method

Everything in the book hangs from a single question. Kant has been circling it: how can pure reason grip objects it never learned about from experience? He now states it in its sharpest form: How are synthetic a priori judgments possible? That sentence is the master key. We unpack it fully in Unit 2; for now, hold onto its shape — it asks how a judgment can be both genuinely informative (synthetic) and known with necessity, in advance of experience (a priori).

This is where the Prolegomena differs most sharply from the Critique, and the difference is one of method. The Critique of Pure Reason uses the synthetic (or progressive) method: it starts from the mind’s basic faculties and builds up, deriving the conditions of knowledge before showing what they make possible. It constructs the cathedral from the foundations. That is rigorous — and it is why the Critique is so hard to enter.

The Prolegomena runs the other way. It uses the analytic — better translated regressive — method. “Analytic” here means analysis in the old sense: start from something already given and admitted, and take it apart to find the conditions it rests on. You begin with a known result and reason backward to its grounds. It is the method of working out what must be true given that something we already accept is true. Prolegomena to Any Future Metaphysics Preface

So what is the “already given and admitted” fact Kant starts from? This is the move that makes the whole book possible. Kant takes it as granted by everyone that we actually possess real bodies of synthetic a priori knowledge — that they exist is not in doubt. His two examples: pure mathematics and pure natural science (the necessary, universal core of physics). We don’t wonder whether 7 + 5 = 12 or whether every event has a cause; we use them, with total confidence, as science.

Now the regressive engine turns. Given that pure mathematics and pure natural science are real synthetic a priori knowledge, Kant reasons backward to the conditions that must hold for them to be possible. Whatever those conditions turn out to be, they are established — because their results are already sitting on the table, undeniable. This is why the Prolegomena can feel like the Critique run in reverse: it assumes the destination in order to uncover the road. Prolegomena to Any Future Metaphysics Preface

Here is the structural payoff. Because Kant’s method works backward from given sciences, the one master question — how are synthetic a priori judgments possible? — splits cleanly into three sub-questions, one per body of knowledge. (1) How is pure mathematics possible? (2) How is pure natural science possible? (3) How is metaphysics in general possible? These three questions are the skeleton of the whole book — and the map of this course.